Magnetic field gradients play a central role in MR imaging. Their functions include encoding spatial information and sensitizing the image contrast to coherent or incoherent motion. Fast, efficient MRI measurements rely on magnetic field gradient waveforms with high temporal fidelity.
Rapid switching of the magnetic field gradients leads to rapidly changing magnetic flux through the rf coil, rf shield, main magnet components and other structures. This changing magnetic flux leads to eddy currents being induced in conducting pathways near the magnet bore. Hardware improvements such as shielded gradient coils and waveform pre-emphasis are largely successful at reducing these effects in modem scanners. The residual eddy currents may however still cause image-quality problems [1] including ghosting in EPI, RARE and GRASE imaging pulse sequences [2], slice-profile modulation with spatial-spectral RF pulses [3], geometric distortion in diffusion-weighted EPI [4], and quantitative velocity errors in phase-contrast imaging [5]. Knowledge of the true gradient waveform in the MRI pulse sequence is critical to addressing and remedying such problems.
Numerous methods have been developed to measure MRI gradient waveforms and k-space trajectories [6-14]. One strategy is magnetic field monitoring with RF microprobes (MFM) [15-16]. Multiple RF microprobes record the magnetic field evolution associated with a wide variety of imaging pulse sequences.
The MFM method involves exciting the sample and measuring the time evolution of magnetization through the FID. However, the gradient waveform duration is limited by the sample T2*. The k-space maxima (i.e. maximum temporal gradient area or image resolution) measurable with MFM are also limited by gradient dephasing. In addition, implementation of this technique is relatively complex as it requires careful probe fabrication, an array of at least 3 probes, accurate probe positioning and alignment and a multi channel receiver.